1. Field of the Invention
The present invention relates to an FM radar system, and more particularly to an FM radar system having a circuit for both modulating a transmission signal or a reception signal and selecting a plurality of beam signals, for thereby increasing the S/N ratio of the reception signal and detecting the bearing of and the distance up to an object with high accuracy.
2. Description of the Prior Art
FM-CW radar systems for use on automobiles operate by frequency-modulating a high-frequency signal of 60 GHz or 77 GHz a millimeter-wave FM signal, transmitting the millimeter-wave FM signal forward of or around the automobile, receiving an echo beam reflected from an object, mixing the received echo signal with a local signal into a beat signal with a mixer, and detecting the position of the object from the frequency of the beat signal.
When the high-frequency signal is frequency-modulated, however, it is difficult to keep the FM signal at a constant level over a sweep frequency range. For this reason, when the high-frequency signal is frequency-modulated, it tends to be amplitude-modulated, and an AM component of the received signal is detected by the mixer.
Since the AM component is substantially the same as the frequency component of the FM signal and has frequencies much lower than those of the transmitted signal, when the AM component is mixed with the beat signal, its frequency is essentially the same as the frequency of the signal indicative of the object, resulting in a reduction in the accuracy with which to detect the position of the object.
The assignee of the present application has proposed an arrangement for avoiding low-frequency noise based on the principles of heterodyne detection as disclosed in Japanese laid-open patent publication No. 4-19588.
Specifically, Japanese laid-open patent publication No. 4-19588 discloses a millimeter-wave radar transmitter and receiver which multiplies the frequency of an FM signal generated by an FM CW source by four, transmits the multiplied-frequency FM signal, and mixes a received echo signal with a local signal which is generated by multiplying the frequency of the FM signal generated by the FM CW source by three, thereby generating a beat signal.
Japanese laid-open patent publication No. 5-40169 reveals an FM-CW radar system which has been proposed to solve the problem of a poor S/N ratio of a received echo signal due to the detection by a mixer of an AM component which has substantially the same frequencies as those of an FM signal and is added to the FM signal because of the gradient of an oscillated-frequency vs. output-voltage curve of a voltage-controlled frequency oscillator for an FM process with a very large frequency deflection. The disclosed FM-CW radar system has a frequency modulator connected between a radar transmitter and a transmission antenna, for modulating a signal, to be transmitted, with a second frequency which is at least twice the sum of or the difference between a Doppler frequency which is much lower than the frequency of the transmitted signal and generated due to a relative speed, and a beat frequency generated owing to a propagation delay time. The signal to be transmitted is modulated by the modulator, and the modulated signal is transmitted from the transmission antenna. A received echo signal reflected from an object is frequency-converted by a local oscillation signal with a first frequency converter, and then frequency-converted by the second frequency with a second frequency converter.
The assignee of the present application has also proposed a time-division multiplex FM radar system as disclosed in Japanese laid-open patent publication No. 7-5252. The proposed time-division multiplex FM radar system comprises a plurality of beam radiators and receivers for radiating overlapping beams of substantially identical radiation patterns and receiving reflected waves, an FM signal generator for generating an FM signal, and a transmitter having a transmission switching circuit for intermittently amplifying a portion of the FM signal at different times and distributing the intermittently amplified signals successively to the beam radiators. The proposed time-division multiplex FM radar system reduces its overall power consumption and interchannel interference, and provides a high detection accuracy over a wide range regardless of the limited number of beam radiators used.
The simplified heterodyne system proposed in Japanese laid-open patent publication No. 5-40169 employs a simpler high-frequency circuit than the heterodyne system proposed in Japanese laid-open patent publication No. 4-19588, but nevertheless offers the same advantages.
However, the simplified heterodyne system is capable of detecting only the bearing of an object because it is not a multiple-beam system. It is important to solve this problem in the application of automobile radar systems because the automobile radar system on an automobile running in a lane of a multiple-lane road needs to have information about adjacent lanes.
The simplified heterodyne system proposed in Japanese laid-open patent publication No. 5-40169 may be combined with the time-division multiplex FM radar system as disclosed in Japanese laid-open patent publication No. 7-5252 to eliminate the problem of the poor S/N ratio of the received echo signal and also to be able to detect position of an object as well as its bearing.
However, if the simplified heterodyne system and the time-division multiplex FM radar system were simply combined with each other, then the combined assembly would require both the modulator for modulating a signal, to be transmitted, in order to improve the poor S/N ratio and the switching circuit for switching transmission beams. Consequently, the combined assembly would result in a complex radar system.